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Seismic Evidence for Large-Scale Compositional Heterogeneity of Oceanic Core Complexes

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Seismic Evidence for Large-Scale Compositional Heterogeneity of Oceanic Core Complexes Article Geochemistry 3 Volume 9, Number 8 Geophysics 6 August 2008 Q08002, doi:10.1029/2008GC002009 GeosystemsG G ISSN: 1525-2027 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Click Here for Full Article Seismic evidence for large-scale compositional heterogeneity of oceanic core complexes J. Pablo Canales and Brian E. Tucholke Department of Geology and Geophysics, Woods Hole Oceanographic Institution, MS 24, 360 Woods Hole Road, Woods Hole, Massachusetts 02543, USA ([email protected]) Min Xu Massachusetts Institute of Technology-Woods Hole Oceanographic Institution Joint Program, Woods Hole, Massachusetts 02543, USA John A. Collins and David L. DuBois Department of Geology and Geophysics, Woods Hole Oceanographic Institution, MS 24, 360 Woods Hole Road, Woods Hole, Massachusetts 02543, USA [1] Long-lived detachment faults at mid-ocean ridges exhume deep-seated rocks to form oceanic core complexes (OCCs). Using large-offset (6 km) multichannel seismic data, we have derived two-dimensional seismic tomography models for three of the best developed OCCs on the Mid-Atlantic Ridge. Our results show that large lateral variations in P wave velocity occur within the upper 0.5–1.7 km of the lithosphere. We observe good correlations between velocity structure and lithology as documented by in situ geological samples and seafloor morphology, and we use these correlations to show that gabbros are heterogeneously distributed as large (tens to >100 km2) bodies within serpentinized peridotites. Neither the gabbros nor the serpentinites show any systematic distribution with respect to along-isochron position within the enclosing spreading segment, indicating that melt extraction from the mantle is not necessarily focused at segment centers, as has been commonly inferred. In the spreading direction, gabbros are consistently present toward the terminations of the detachment faults. This suggests enhanced magmatism during the late stage of OCC formation due either to natural variability in the magmatic cycle or to decompression melting during footwall exhumation. Heat introduced into the rift valley by flow and crystallization of this melt could weaken the axial lithosphere and result in formation of new faults, and it therefore may explain eventual abandonment of detachments that form OCCs. Detailed seismic studies of the kind described here, when constrained by seafloor morphology and geological samples, can distinguish between major lithological units such as volcanics, gabbros, and serpentinized peridotites at lateral scales of a few kilometers. Thus such studies have tremendous potential to elucidate the internal structure of the shallow lithosphere and to help us understand the tectonic and magmatic processes by which they were emplaced. Components: 10,396 words, 13 figures. Keywords: oceanic core complex; detachment fault; Mid-Atlantic Ridge; seismic structure; gabbro; serpentinized peridotite. Index Terms: 3025 Marine Geology and Geophysics: Marine seismics (0935, 7294); 3035 Marine Geology and Geophysics: Midocean ridge processes; 3036 Marine Geology and Geophysics: Ocean drilling. Received 28 February 2008; Revised 10 June 2008; Accepted 27 June 2008; Published 6 August 2008. Copyright 2008 by the American Geophysical Union 1 of 22 Geochemistry 3 canales et al.: heterogeneity of oceanic core complexes Geophysics 10.1029/2008GC002009 Geosystems G Canales, J. P., B. E. Tucholke, M. Xu, J. A. Collins, and D. L. DuBois (2008), Seismic evidence for large-scale compositional heterogeneity of oceanic core complexes, Geochem. Geophys. Geosyst., 9, Q08002, doi:10.1029/2008GC002009. 1. Introduction (MAR): Kane [Dick et al., 2008; Tucholke et al., 1998], Dante’s Domes [Tucholke et al., 2001], and [2] Oceanic core complexes (OCCs) are deep sec- Atlantis [Cann et al., 1997] (Figure 1). The to- tions of the oceanic lithosphere exhumed to the mography models reveal large lateral variations in seafloor by long-lived detachment faults [Cann et P wave velocity within the shallow lithosphere al., 1997; MacLeod et al., 2002; Tucholke et al., beneath all three OCCs, as well as distinctly 1998]. When active, these faults constitute the sole different velocity structure in the associated volca- extensional boundary between separating tectonic nic hanging walls. A strong correlation between the plates [deMartin et al., 2007; Smith et al., 2006; modeled velocity variations and lithology of in situ Tucholke et al., 1998], and in some instances they geological samples allows us to constrain the large- can accommodate extension for 1–2 Ma forming scale spatial distribution of gabbroic intrusions and relatively smooth-surfaced, dome-shaped mega- serpentinized peridotites within the OCCs, thus mullions due to footwall rollover [Tucholke et al., providing valuable new insights into the formation 1996, 1998]. These features commonly exhibit and evolution of these features. spreading-parallel corrugations or ‘‘mullions’’ that can have amplitudes up to several hundred meters, 2. Geological Settings and they have been identified at a wide variety of spreading centers [Cannat et al., 2006; Ohara et [5] The three OCCs studied show characteristic al., 2001; Okino et al., 2004; Searle et al., 2003; smooth surfaces and spreading-parallel corruga- Tucholke et al., 1998, 2008]. tions typical of megamullions formed by major oceanic detachment faults [Tucholke et al., 1998]. [3] The abundance of gabbros and peridotites Hereinafter we use Tucholke et al.’s [1998] recovered in most OCCs [Blackman et al., 2002, definitions of breakaway zone (where the faults 2006; Cann et al., 1997; Dick et al., 2000, 2008; initially nucleated) and termination of the faults. Escartı´n et al., 2003; MacLeod et al., 2002; Reston Breakaway zones are generally defined by iso- et al., 2002; Tucholke and Lin, 1994], together with chron-parallel ridges that mark the older limit of gravity modeling that suggests the presence of fault corrugations; however, early corrugations mantle-like densities near the seafloor [e.g., often are not well developed and this limit can be Nooner et al., 2003], indicates that OCCs may be difficult to determine. Terminations are more easily ideal tectonic windows where the geological record identified because they mark the young limit of a of mantle flow and melt generation and migration smooth, corrugated fault surface against rougher can be studied [Tucholke et al., 1998]. They also adjacent terrain of the hanging wall. have the potential for hosting serpentinite-based hydrothermal systems [Fru¨h-Green et al., 2003; Kelley et al., 2001]. However, the relative abun- 2.1. Kane Oceanic Core Complex dances and distribution of these lithologies are not [6] Kane OCC is located between 30 and 55 km well constrained. Drilling at OCCs to date has off-axis on the North American plate immediately recovered dominantly gabbroic rocks [Blackman south of the Kane Transform Fault (TF), between et al., 2006; Dick et al., 2000; Kelemen et al., 23°200Nand23°400N (Figure 1). It formed 2004], raising a question of the extent to which between 3.3 Ma and 2.1 Ma under conditions of OCCs actually provide access to the oceanic man- strongly asymmetrical spreading (17.9 mm aÀ1 on tle. Thus the origin and evolution of OCCs, their the OCC side to the west, and 7.9 mm aÀ1 on the significance as tectonic windows into the oceanic conjugate between Chrons 2 and 2A [Williams et lithosphere, and their potential as widespread hosts al., 2006]). The breakaway for this OCC is marked of serpentinite-based hydrothermal ecosystems re- by a linear ridge that is continuous for more than main uncertain. 35 km along isochron (with the possible exception of a small offset near latitude 23°300N, Figure 1). [4] In this paper we present seismic tomography The detachment fault constituting the surface of the images obtained across three of the best developed OCC exhibits several domes (Babel, Abel, Cain, and best studied OCCs on the Mid-Atlantic Ridge 2of22 Geochemistry 3 canales et al.: heterogeneity of oceanic core complexes Geophysics 10.1029/2008GC002009 Geosystems G Figure 1. Shaded-relief, three-dimensional perspectives of multibeam bathymetry over Atlantis, Kane, and Dante’s Domes OCCs. (Note the different view angle of the Dante’s Dome image). Solid lines locate seismic profiles (numbered with A, DD, and K). Dashed lines show locations of breakaways and terminations of detachment faults. SR and CD are the Southern Ridge and Central Dome, respectively, of Atlantis OCC; ND and SD are the northern and southern domes, respectively, of Dante’s Domes OCC; and AD, BD, CD, and ED are the Abel, Babel, Cain, and Eve domes, respectively, of Kane OCC. Yellow star on Atlantis OCC locates the Lost City hydrothermal field. The inset at left shows the location of the three OCCs on the flanks of the MAR in the northern central Atlantic. Adam, and Eve, following the nomenclature of serpentinized peridotite at the center of the OCC, Dick et al. [2008]) and is cut by a large-offset, providing direct evidence of the footwall compo- high-angle, west-facing normal fault (East Fault, sition there [Dick et al., 2008]. labeled in Figure 1). Extensive geological sampling indicates that the central and western parts of the 2.2. Dante’s Domes Oceanic Core Complex Kane OCC are predominantly ultramafic [Dick et [7] Dante’s Domes OCC lies just east of the MAR al., 2008]. Both peridotites and gabbros are ex- rift valley on the African plate, between 26 320N posed along the northern edge of the OCC (south ° and 26 400N, away from any major segment wall of the Kane TF) [Auzende et al., 1994]. Slide ° discontinuity [Tucholke et al., 2001] (Figure 1). scars along East Fault expose massive outcrops of Total spreading rate in the area during formation of 3of22 Geochemistry 3 canales et al.: heterogeneity of oceanic core complexes Geophysics 10.1029/2008GC002009 Geosystems G the OCC was 22 mm aÀ1 [DeMets et al., 1990]. towed at a nominal depth of 10 m. Hydrophone A set of ridges near the eastern limit of the OCC group (i.e., receiver) spacing was 12.5 m.
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